Liquid level sensor

ABSTRACT

The level of a liquid is detected by an ultrasonic arrangement comprising a transmitter and a receiver. The transmitter has a vertical array of transmitter segments which transmit sequentially, while the receiver is a single transducer. The transmitter and receiver are formed as a single assembly operating in pulse echo mode in conjunction with a passive reflector. The difference in transmission time in liquid and gas allows a digital, depth representative output to be generated. The transmitter is formed by securing a single sheet of piezo polymer film over a printed circuit board having a conductive pattern defining the segments.

FIELD OF THE INVENTION

This invention is relates to a device for sensing the level of liquidwithin a container and, particularly, to a device in the form of a solidstate arrangement having no moving parts and readily adaptable to dealwith containers of non-uniform shape.

SUMMARY OF THE INVENTION

There are applications where it is desirable to determine liquid levelas a discrete, rather than continuous, function. An example of such acase is where the final display of level is desired to be a digitalreadout. Certain forms of the present invention provide a device whichfulfils this function by providing a plurality of independent ultrasonictransducers disposed in such a way that the presence or absence ofliquid between a given transmitter element and a given receiver elementcan readily be detected.

The invention and preferred features thereof are defined in the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic cross-sectional view of a liquid level sensorforming a first embodiment of the invention;

FIG. 2a is a cross-sectional side view of a second embodiment of sensor;

FIG. 2b is a sectional plan view of the sensor of FIG. 2a;

FIG. 3 is a graph illustrating the operation of one aspect of the sensorof FIG.2;

FIG. 4a shows a pcb track layout used in a third embodiment;

FIG. 4b is an enlarged view of part of FIG. 4a;

FIG. 4c is a cross-sectional side view of an embodiment using the pcb ofFIG. 4a;

FIG. 5 illustrates a two-segment receiver of a second embodiment, thereceiver having equal but opposite receiver segments;

FIG. 6 shows the ratiometric output of the receiver of FIG. 5;

FIGS. 7a and 7b illustrate a two-segment receiver similar to that ofFIG. 5 but having segments defined by a non-uniform curve;

FIG. 8 shows signal strengths from the segments of the receiver of FIG.7a;

FIG. 9 shows the ratiometric output of the receiver of FIG. 7a and

FIG. 10 illustrates one form of electronics suitable for use with thereceiver of FIG. 7a.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a sensor comprises a pair of ultrasonic transducers10, 12 mounted at a fixed separation S on either side of a containerholding liquid 14, or directly immersed in liquid. One transducer 10forms a transmitter and comprises a plurality of independent conductive(i.e., transmitter) segments 16 each of which may be individuallyaddressed by associated drive electronics (not shown). The othertransducer 12 forms a receiver and comprises a single conductive element18 having a sufficient geometric extent to receive ultrasonic acousticsignals generated by activation of any of the transmitter segments 16.

The device of this embodiment is used in the following manner. Anelectrical stimulus is applied to each transmitter segment 16 in turn.The time delay between application of the stimulus and reception of theresulting acoustic signal may be calculated or measured for the specificliquid involved, and is governed by the speed of sound in the medium andby the physical separations of the transmitter 10 and receiver 12. If asignal has not been detected by the receiver 12 within the appropriatetime window, then it can be concluded that the liquid has not reachedthe height of the given transmitter segment.

The above description, involving a plurality of independent segments 16for the transmitter, would be difficult and costly to realize usingphysically separate piezoelectric elements.

It is a preferred feature of the invention, therefore, that the mostsuccessful reduction to practice will employ a continuous piezoelectricmatrix 20 with one common electrical ground connection 22, together witha plurality of segments 16. (It is not necessary for the groundelectrodes for each segment 16 to be separate.)

This can be accomplished if the mechanical coupling of acoustic energyis poor between closely-spaced electrodes: i.e. if the internal dampingof the piezoelectric material 20 is great. One suitable piezoelectricmaterial is suitably processed poly(vinylidene fluoride) (PVDF or PVF₂);another is vinylidene fluoride-trifluoroethylene (VF₂ VF₃) copolymer.

Suitable patterning of the signal electrodes may be accomplished by thenormal means, i.e. by vacuum depositing or sputtering the metallicelectrode material through a mask, or by etching a continuous metalliclayer to obtain the required pattern.

It is a further preferred feature of the invention, however, that therequired patterning of one or both transducers may be formed in asimpler way: a continuous piezoelectric matrix may be laid down upon apatterned substrate. In particular, the patterned substrate may consistof printed circuit board (p.c.b.) 24 whereby etched lands of copper mayform the segments 16. This method has the benefits of simple and robustconstruction using standard p.c.b. techniques, and avoids therequirement of a plurality of connections to be made to the somewhatdelicate metallization which can be deposited on the piezoelectricpolymer.

An additional benefit of using p.c.b. material as the backing layer isthat double-sided beard may be used. Plated-through holes 26 then allowinterconnection and wiring of all segments to be accomplished on therear surface connection pads 28, leaving the front surface unhinderedfor application of the piezoelectric layer 20. A further benefit ofusing double-sided circuit board is that the rear side copper may beleft substantially intact as shown at 30, and connected to groundpotential, where the receiver design is considered. This affords goodelectrical shielding of the receiver element, so reducing the extent ofthe electromagnetically-coupled signal from the transmitter. It is ofgreat importance that this "cross-talk" is minimised, to avoidsaturation of the receiver circuitry, or false triggering, prior to thearrival of the true acoustic signal.

The piezoelectric polymer matrix 20 may be applied in two ways. Onemethod is to use a sheet of said polymer, with metallization applied tothe outer surface only. The inner surface may be bonded to the patternedp.c.b. material 24 by a number of adhesive methods, for instance, usingepoxy resin. The adhesive layer so introduced is normally thin inrelation to the thickness of the piezoelectric polymer employed. Theelectrical signal applied to or generated by such an assembly will becapacitively coupled through the additional dielectric layer formed bythe adhesive.

An alternative method of applying the piezopolymer matrix can beemployed where the piezoelectric material used is the VF₂ VF₃ copolymermentioned above. This material may initially consist of a solution whichcan be applied by casting or spin-casting directly on the p.c.b.material. After evaporation of the solvent, the resulting layer mustthen be polarized as part of the process whereby it is renderedpiezoelectric: this polarization may be preferably applied only at theregions of the signal electrodes. Thus, only the polymer material lyingabove each patterned electrode may be considered to be piezolectric,while the material between each electrode may be considered inert.Suitable polarization procedures are well known to those in the art.

The control circuitry associated with the device of the foregoingembodiment is required to fulfil the following functions:

1) to generate an electrical signal of sufficient amplitude andfrequency to create a useful acoustic signal from the transmittersegments.

2) to apply the abovementioned electrical signal to each transmittersegment in turn.

3) to amplify and threshold the receiver signal

In addition, the following functions may be desirable:

4) to generate timing pulses to gate the transmitter signal since only ashort burst is required

5) to generate other timing pulses to gate the receiver, in such a wayas to prevent any other signal than the possible arrival of the acousticwave being amplified

6) to provide counting and logic to allow generation of a numericaloutput corresponding to the "index number" of the appropriate segmentbeing the last to indicate presence or absence, if "top downwards" logicis used or fluid.

The system is capable of a self-check function. It is possible to drivethe piezoelectric transducers at such a frequency that the acousticsignal may be transmitted successfully through air as well as liquid,although with different transmission delay. In this case, the operationof all segments may be verified periodically.

In a preferred form of the embodiment described, the electrical signalapplied to the transmitter has substantial energy content in thefrequency range 200 kHz to 5 MHz. The time duration of the appliedsignal may vary widely, between a very fast spike and a step function ofindeterminate length. A rectangular pulse of 0.1 to 10 μs is preferred,especially about 0.2 μs. The spacing S between the transmitter 10 andthe receiver 12 may suitably be between 1 mm and 25 mm. Thepiezoelectric films 20 may suitably have a thickness of 0.1 μm to 500μm; it his been found however that the electrical behaviour issubstantially independent of film thickness and any available filmthickness will produce a usable result.

A possible modification of the foregoing embodiment of the presentinvention would use a receiver transducer of the same construction asthe transmitter; in other words, the system would comprise two identicallinear arrays, with the electrical stimulus being applied to eachtransmitter segment in turn, while the corresponding receiver segment ismonitored for the arrival or non-arrival of the acoustic signal. Thisembodiment offers greater electrical signal strength of the detectedreceiver signal since, in effect, the entire area of each receiversegment may be excited by the incidence of the acoustic wave, but thismethod increases the complexity of the associated electronic circuitry.

It is envisaged that the height of each transmitter segment 16 be small,to allow a large number of segments within the range of the sensor, andthat the spacing between each segment be either regular or varied,according to the geometry of the container.

A further optional feature of the invention is that the propagationdelay of the acoustic signal from transmitter to receiver may bemeasured, and so knowledge obtained regarding the composition of theliquid. An example of this application would be when a possibilityexists of contamination of gasoline with water, where the water forms alayer at the bottom of the container. Where the speeds of sound aresufficiently distinct, and where a suitably short acoustic pulse orburst of pulses can be sent, then appropriate control signals can begenerated to detect abnormal early reception of the acoustic signal. Thespeed information can be examined for each segment in turn, and thus aprofile of the characteristics of the liquid column can be developed.

The digital nature of the output produced by the foregoing embodimentlends itself to further digital transmission and processing. As oneexample, each level may have a corresponding weighting factor appliedfrom a look-up table held in a ROM or the like, to provide a user outputin terms of liquid volume rather than liquid depth. This is particularlyuseful in monitoring the contents of tanks having irregular stapes.

FIG. 2 illustrates a modified version of the embodiment of FIG. 1. Onceagain, the transducers 10, 12 comprise pcb's 24 carrying piezo films 20.These are mounted within a housing in the form of an extruded polymertube 32 which is placed within the tank (not shown). The electroniccomponents 34, which may suitably be surface mount devices, are mounteddirectly on the conductor patterns on the rear side of the transmitterpcb 24.

To afford the components 34 protection from chemical attack,electrolytic corrosion and the like, the cavities formed between thetube 32 and the pcb's 24 are filled with an encapsulation material 36such as epoxy resin. The piezo films 20 are less prone to attack, butthe metallized surfaces may be chemically attacked or corroded.Protective coatings 38 are therefore provided. The coatings 38 may beconformal coatings of suitable protective material, for example epoxyresin, applied in any suitable way such as dipping, brushing orspraying. Alternatively, the coatings 38 may be provided by bonding athin sheet of inert, stable polymer onto the front face of thetransducer in such a way that, as shown in FIG. 2, the polymer sheetitself acts to locate the assembly within the tube 32.

Said suitable inert polymers may be, for example only, polyethylene(UHMW or HDPE) or polyvinylidene fluoride (PVDF, Kynar), or others.

Some effect on the acoustic transmission is expected from theapplication of a front-face layer--it is found, in general, that thebandwidth of the ultrasonic device so formed is somewhat reduced.Thicknesses of 0.5 mm to 1.0 mm of HDPE have been found to allowsatisfactory operation. It is a feature of the piezopolymers employed(PVDF or VF₂ VF₃) that their acoustic impedance closely matches thosematerials found to be suitable as protective layers. Thus the transferof acoustic energy remains acceptable.

The use of an extruded tube 32 (which may be circular or rectangular insection) allows the possibility of forming it with mounting grooves orribs for the pcb's and other assembly aids.

As also seen in FIG. 2, the housing is substantially closed by end caps40, 42 on the tube 32. The lower end cap 40 is provided with arestricted aperture 44 for entry and exit of the liquid, and an airbleed aperture 46 is provided in the upper end cap 42. This provides adamping effect on the rate of liquid level change and, especially inconjunction with a narrow spacing between the transducers, greatlylimits the effects of liquid surging or "slosh". If necessary, theinternal cavity can be further provided with baffles (not shown).

A further optional feature illustrated in FIG. 2 is a self-testcapability. A block 50 of solid material is disposed between thetransducers 10, 12 in such a manner that it links part of eachtransmitter segment directly to the receiver. With this arrangement, thetransmission of a pulse from a transmitter segment causes a pulse to bereceived via the block 50 before any transmission through the liquid.This is illustrated in FIG. 3, in which (a) represents a short, unipolartransmitted pulse, (b) represents the receiver signal in air, with adistinct pulse being received only via the solid block, and (c)represents the receiver signal in water, the second distinct pulse beingthe transmission via the water. Suitable materials for the block 50 arepolymers such as high-density polyethylene and polymethylmethacrylate.The example of FIG. 3 was measured with a PMMA spacer of 14.6 mmthickness covering 50% of each transmitter segment.

This feature can be used to give warning of a system failure, since innormal operation a pulse will be received a given time after everytransmitted pulse.

In a further modified embodiment, the spaced transmitter and receivertransducers are replaced by a single transmitter/receiver assemblyoperating in pulse-echo mode in conjunction with a passive reflector. Itis known in other applications of ultrasonics to use pulse-echotechniques. Conventionally, this requires a receiver to be switched atappropriate times to avoid it detecting the transmitted pulse. In thepresent application with a short transmission path and a number oftransmitters, such switching would be possible but would be complex andexpensive to implement.

However, a pulse-echo form of the present invention can be implementedin a simpler manner, as will now be described. FIG. 4a illustrates aprototype transmitter/receiver assembly incorporating six transmittersegments and a common receiver. As in the above embodiments, theassembly makes use of piezo film secured to a pcb. FIG. 4a shows thetrack layout of the pcb, while FIG. 4b shows part of the track layout ingreater detail.

Each transmitter segment 60 is formed by a pair of spaced strips 60a,60b interleaved by a receiver strip 62. The receiver strips areconnected together at 64, while each transmitter segment 60 is connectedindependently to the associated circuitry by a via 66. The overalleffect is therefore a series of transmitter segments, and a commonreceiver segment of large area, but comprising an array of interdigitalparts.

The transmitter segments 60 are fired individually in the same manner asin the embodiments of FIGS. 1 and 2, and the same receiver processing isused. Instead of a separate pcb, all that is required is a passivereflector (which could be a separate item, or simply the wall of thetank) at a suitable spacing to give sufficient time delay for anydirectly coupled crosstalk to subside. To minimise breakthrough of thedrive signal from transmitter to receiver electrodes, a continuousground conductor 68 is preferably interleaved between them. It is alsopreferable to interlay a plane of cooper which is substantially unbroken(except for the vias such as 66 carrying signals through this layer tothe component side) which serves as a ground plane. Thus, the receiverstrips 62 will see a ground plane to the rear (the mid-board copper), toeach side (the conductor 68), and also to the front (the piezo filmmetallization).

FIG. 4c shows an embodiment of the invention, in which the sensor ofFIG. 2 is modified to make use of the pcb of FIGS. 4a and 4b. In FIG.4c, the pcb 24 operates in conjunction with a passive reflective surface47. The sensor is otherwise similar to that of FIG. 2 with likereferences.

The convenience and accuracy of the foregoing systems are clear, butsituations may arise where an inherently digital output is not required.As mentioned above, the digital output may be converted into analogueform, with some increase in complexity. It is also possible, however, touse similar construction techniques and materials to provide anaccurate, analogue measurement.

Turning to FIGS. 5 to 10, consider first a simple ultrasonictransmitter/receiver system where both transducers are plain,rectangular elements, formed by bonding piezoelectric polymer film ontoa suitable substrate e.g. printed circuit board material. As the liquidlevel between the two plates rises, the acoustic coupling of signal willincrease in a linear fashion from zero to a maximum value.

This linear response may be converted easily into an analogue voltage.However, any change in sensitivity of the two transducers will createproblems in calibration. The piezo coefficients of the polymer are knownto change with temperature, and the required range is broad for certainapplications (-40°to +85° C. is typical for automotive).

It is thus desirable to develop a method such that the final output issubstantially independent of the absolute value of the received signal.

Consider next a similar system, but where the rectangular receiverelement is split diagonally into two independent, triangular receiverareas A and B as shown in FIG. 5. The transmitter is once again a solidrectangle, mounted in opposition to the receiver. As the fluid levelrises in such an assembly, we can examine the ratio of the output signalamplitude of receiver A to receiver B, which will increase over therange of fluid height. The curve will not be linear, but approximatelyquadratic, as in FIG. 6. It would be possible to "linearise" such afunction electronically, but it is also possible to pattern thereceivers to arrange a linear ratio function.

It can be shown that a specific function

    f(x)=(2x/(1+x))-(x.sup.2 /(1+x).sup.2)

where x represents the varying immersion depth describes a curve whichsplits the rectangular receiver area into two portions, each of equalarea, such that the ratio of their areas immersed will change from zeroto unity in a linear function of depth.

The "area function" of one receiver segment is given by the integral off(x):

    g(x)=f(x)dx=x.sup.2 /(1+x)

while the other receiver segment has area x-g(x).

The signal output from each receiver is found to be directlyproportional to the "area function" g(x).

FIG. 7a and 7b (referred to collectively as FIG. 7) illustrate suitableelectrode patterns, the transmitter electrode being shown at 110 and thesplit receiver electrodes at 112 and 114. FIGS. 8 and 9 show graphicallythe absolute signal strength and signal strength ratio producedexperimentally by the transmitter and receiver of FIG. 7. Theratiometric output shows good approximation to a linear response.

Note that for the extreme "empty" condition, both the dividend and thedivisor tend to zero, and so the electronic output will be undefined.Several methods may be adopted to prevent malfunction. For example, theabsolute value of the "reference channel" (divisor) signal strength maybe examined, and appropriate action taken if it lies below the detectionthreshold. The output may be blanked or simply indicated as "low level".

Alternatively, a small additional receiver segment extending from zeroto critical depth may be patterned, such that the division is onlyperformed if this segment is "fully on".

One example of the electronics required for a basic measurement systemusing the sensor of FIG. 7 is shown in FIG. 10. The transmitter 10 isdriven by an oscillator 46 which may supply a constant or burst signalat suitable frequency, typically 0.5-1.5 MHz, and moderate amplitude,typically 1-20 volts peak-to-peak. The outputs of the receiver segments112, 114 are fed via dual, matched gain stages 118A and 118B to dualpeek detectors 120A, 120B to give absolute signal strength inputs toanalog divider circuit 122. The peak detectors 120 may have short holdtimes, or alternatively RMS-to-DC converters may be used. The analogdivider 122 may suitably be AD532, AD534 or AD538. A threshold detector124 operates gate 126 to blank the output when the received signal isclose to zero.

In a modified form of the foregoing embodiment, the curve defining eachreceiver segment may be adjusted to compensate for non-uniformity of thecontainer cross-section, such that the relationship between ratiometricsignal output and liquid volume remains constant.

I claim:
 1. A liquid level sensor device comprising a transmitter and areceiver of ultrasonic energy, the transmitter for transmitting pulsesto the receiver along a transmission path of known length, and adaptedto be positioned in a liquid container such that liquid in the containeris present in the transmission path, the transmitter comprising aplurality of individually addressable independent segments spaced apartin the direction of liquid depth, the segments comprising conductivesignal electrodes, where the transmitter and receiver are formed as asingle assembly operating in pulse-echo mode in conjunction with apassive reflector in the transmission path.
 2. A sensor device accordingto claim 1, wherein the transmitter and receiver comprise apiezoelectric film mounted on a printed circuit board (pcb).
 3. A sensordevice according to claim 2, wherein the pcb has a track layout thatcomprises the signal electrodes.
 4. A device according to claim 3,wherein the piezoelectric film is made of poly vinylidene fluoride orvinylidene fluoride-trifluoroethylene copolymer.
 5. A device accordingto claim 1, in which the depthwise spacing between segments isnon-uniform.
 6. A device according to claim 2, in which thepiezoelectric film is protected from the liquid by an overlyingprotective coating or sheet.
 7. A device according to claim 1, in whichthe transmitter and receiver are mounted within a tube for immersion ina tank.
 8. A device according to claim 7, in which the tube issubstantially closed except for restricted orifice means which limit therate of liquid flow into and out of the tube.
 9. A liquid level sensordevice comprising a transmitter and receiver of ultrasonic energy; thetransmitter for transmitting pulses to the receiver along a transmissionpath of known length, and adapted to be positioned in a liquid containersuch that liquid in the container is present in the transmission path,the transmitter comprising a plurality of individually addressableindependent segments spaced apart in the direction of liquid depth, thesegments comprising conductive signal electrodes, wherein thetransmitter comprises a patterned substrate, and a piezoelectric matrixmounted thereon.
 10. A sensor device according to claim 9, wherein theconductive signal electrodes are mounted on the substrate.
 11. A sensordevice according to claim 10, wherein the transmitter comprises a singlecontinuous said piezoelectric matrix.
 12. A device according to claim11, in which the piezoelectric matrix is made of polymeric material. 13.A device according to claim 9, in which the depthwise spacing betweensegments is non-uniform.
 14. A device according to claim 9, in which thepiezoelectric matrix is protected from the liquid by an overlyingprotective coating or sheet.
 15. A device according to claim 9, in whichthe patterned substrate is a printed circuit board (pcb).
 16. A deviceaccording to claim 15, in which the pcb is copper clad on both sides.17. A device according to claim 16, wherein one side of the pcb hassubstantially continuous copper serving as electrical shielding.
 18. Adevice according to claim 16, in which the copper cladding of one sideof the printed circuit board is formed into said segments to whichconnection are made by plated-through holes.
 19. A device according toclaim 16, in which one side of the printed circuit board bearsconductive pattern defining said segments and the other side of theprinted circuit board bears a conductive pattern for direct mounting ofassociated circuit components.
 20. A device according to claim 11, inwhich the transmitter and receiver are mounted within a tube forimmersion in a tank.
 21. A device according to claim 20, in which thetube is substantially closed except for restricted orifice means limitedto the rate of liquid flow into and out of the tube.
 22. A liquid levelsensor device comprising a transmitter and receiver of ultrasonicenergy, the transmitter for transmitting pulses to the receiver along atransmission path, the device to be positioned in a liquid containersuch that liquid in the container is present in the transmission path,wherein the receiver comprises a piezoelectric material patterned toform two independent active area segments, each segment having a widththat varies along the liquid height in a manner differing from the othersuch that the ratio of signal strengths of one segment with respect tothe other varies dependent on the liquid height.
 23. A device accordingto claim 2, in which the widths of the segments vary in such a mannerthat the ratio of signal strengths increases substantially as a linearfunction of liquid height.
 24. A device according to claim 22, includinga further receiver segment adjacent to zero liquid level for producing asignal for controlling the determination of the ratio of signalstrengths.
 25. A device according to claim 22 including a thresholddetector connected to one of the receiver segments to produce a signalindicating that liquid level is below a detection threshold.